Autore: ExtremeTech

An Asus 802.11ac router

Over the last few weeks, the first round of 802.11ac WiFi devices have started to emerge. In essence, 802.11ac is a supercharged version of 802.11n (the current WiFi standard that your smartphone and laptop probably use), offering link speeds ranging from 433 megabits-per-second (Mbps), all the way through to multiple gigabits per second. To achieve speeds that are dozens of times faster than 802.11n, 802.11ac works exclusively in the 5GHz band, uses a huge wad of bandwidth (80 or 160MHz), operates in up to eight spatial streams (MIMO), and a utilizes very fancy technology called beamforming. For more details on what 802.11ac is, and how it will eventually replace wired gigabit ethernet networking at home and in the office, read on.

How 802.11ac works

At its core, 802.11ac is essentially an updated version of 802.11n, which itself introduced some very exciting technologies that brought massive speed boosts over 802.11a and g. Whereas 802.11n had support for four spatial streams (4×4 MIMO) and a channel width of 40MHz, 802.11ac can use eight spatial streams and has channels up to 80MHz wide, which can be combined to make 160MHz channels. Even if everything else remained the same (it doesn’t), this means that 802.11n has 8x160MHz of spectral bandwidth to play with, vs. 4x40MHz — a huge difference that allows 802.11n to squeeze vast amounts of data across the airwaves.

WiFi 802.11acTo boost throughput further, 802.11ac also introduces 256-QAM modulation (up from 64-QAM in 802.11n), which basically squeezes 256 different signals over the same frequency by shifting/twisting each signal to a slightly different phase. In theory, this quadruples the spectral efficiency of 802.11ac over 802.11n. Spectral efficiency is a measure of how well a given wireless protocol/modulation/multiplexing technique uses the bandwidth available to it. In the 5GHz band, where channels are fairly wide (20MHz+), spectral efficiency isn’t so important; in the cellular bands, though, channels are often only 5MHz wide, which makes spectral efficiency very important.

802.11ac also introduces standardized beamforming (802.11n was non-standardized, which made interoperability an issue). Beamforming is essentially transmitting radio signals in such a way that they’re directed at a specific device. This can increase throughput (and make throughput more predictable), and also reduce power consumption. Beamforming can be done with smart antennae that physically move to track the device, or by modulating the amplitude and phase of the signals so that they destructively interfere with each other, leaving just a narrow, not-interfered beam. 802.11n uses this second method, which can be implemented by both routers and mobile devices.

Finally, 802.11ac is fully backwards compatible with 802.11n and 802.11g — so you can buy an 802.11ac router today, and it should work just fine with your older WiFi devices.

The range of 802.11ac

In theory, at the 5GHz band and using beamforming, 802.11ac should have the same or better range than 802.11n (without beamforming). The 5GHz band, due to less penetration power, doesn’t have quite the same range as 2.4GHz (802.11b/g), but that’s the trade-off we have to make: There simply isn’t enough spectral bandwidth in the massively overused 2.4GHz band to allow for 802.11ac’s gigabit-level speeds. As long as your router is well-positioned, or you have multiple routers, it shouldn’t matter a huge amount. (Personally, two 802.11n routers, placed on strategic windowsills, are more than enough to cover my massive house and gardens with a high-quality WiFi signal.)

As always, the more important factor will likely be the transmission power of your devices, and the quality of their antennae.

Trendtech 802.11ac router

How fast is 802.11ac?

And finally, the question that’s on everyone’s lips: Just how fast is WiFi 802.11ac? As always, there are two answers: the theoretical max speed that can be achieved in the laboratory, and the practical max speed that mere mortals will receive at home, surrounded by lots of signal-attenuating obstacles.

The theoretical max speed of 802.11ac is eight 160MHz 256-QAM channels, each of which are capable of 866.7Mbps — a grand total of 6,933Mbps, or just shy of 7Gbps. That’s a transfer rate of 900 megabytes per second — more than you can squeeze down a SATA 3 link. In the real world, due to channel contention, you probably won’t get more than two or three 160MHz channels, so the max speed comes down to somewhere between 1.7Gbps and 2.5Gbps. Compare this with 802.11n’s max theoretical speed, which was 600Mbps.

In practice, the current max speed of 802.11ac devices is 1.7Gbps, because there doesn’t appear to be any devices on the market that can bond two 80MHz channels into 160MHz. Hard data is hard to come by, but it seems we’ll have to wait for the second wave of 802.11ac devices — due in 2014, after the standard is finalized — before 160MHz channels and multi-gigabit speeds become a reality. The max speed over an 80MHz channel is 433.3Mbps, and there aren’t any 802.11ac chipsets that support more than four streams. Again, the next wave of devices should up this to eight streams.

2013 Apple Airport Extreme, disassembled by iFixit

An 802.11ac Apple Airport Extreme, disassembled by iFixit

In reality, the best you can currently do is the 2013 Apple Airport Extreme or the Western Digital My Net AC1300, both of which support three streams, for a total of 1.3Gbps of bandwidth. As there aren’t currently any smartphones, tablets, or laptops on the market that support more than two streams, though, 1.3Gbps remains a pipe dream — for now.

In Anandtech’s testing, they paired a WD MyNet AC1300 802.11ac router (up to three streams), paired with a range of 802.11ac devices that supported either one or two streams. The fastest data rate was achieved by a laptop with an Intel 7260 802.11ac wireless adapter, which used two streams to reach 364 megabits per second — over a distance of just five feet (1.5m) At 20 feet (6m) and through a wall, the same laptop was the fastest — but this time maxing out at 140Mbps. The listed max speed for the Intel 7260 is 867Mbps (2x433Mbps streams). In Geek.com’s testing, an HTC One (which is capable of single-stream 802.11ac) manages 120Mbps to 2013 Apple AirPort Extreme — but it probably could’ve gone faster, if Geek hadn’t bottlenecked itself by performing the download test over 150Mbps Verizon FiOS.

In general, then, you can certainly expect some impressive speeds from 802.11ac, but it won’t replace your wired Gigabit Ethernet network just yet. In situations where you don’t need the maximum performance and reliability of wired GigE, though, 802.11ac is very compelling indeed. Instead of cluttering up your living room by running an Ethernet cable to the home theater PC under your TV, 802.11ac now has enough bandwidth to wirelessly stream the highest-definition content to your HTPC. For all but the most demanding use cases, 802.11ac is a very viable alternative to Ethernet.

The future of 802.11ac

802.11ac will only get faster, too. As we mentioned earlier, the theoretical max speed of 802.11ac is just shy of 7Gbps — and while you’ll never hit that in a real-world scenario, we wouldn’t be surprised to see link speeds of 2Gbps or more in the next few years. At 2Gbps, you’ll get a transfer rate of 256MB/sec, and suddenly Ethernet serves very little purpose indeed.

To reach such speeds, chipset and device makers will have to actually suss out how to implement four or more 802.11ac streams, both in terms of software and hardware. We’re sure that Broadcom, Qualcomm, MediaTek, Marvell, and Intel are already well on their way to implementing four- and eight-stream 802.11ac solutions for integration in the latest routers, access points, and mobile devices — but until the 802.11ac spec is finalized in early 2014, second-wave chipsets and devices are unlikely to emerge. A lot of work will be have to done by the chipset and device makers to ensure that advanced features, such as beamforming, comply with the standard and are interoperable with other 802.11ac devices.

Now read: What is LTE? and, What is LTE-Advanced?